Computerized color graphics systems and electronic printing systems are known in the art. Typically, they enable a user to produce a color image, e.g., any visual two-dimensional pattern including text, graphic line art, continuous tone images, etc., and from that image to produce a picture which can be printed using a color reproduction system, for example, by producing color separation plates for offset printing. There has been much effort in the past to develop ways to determine these “separations”, especially for multi-ink systems, e.g. systems where more than four inks are being used, as is for example the case for very high quality color printing and in textile or decoration printing.
Printing may be carried out using halftoning, also called screening, which is the process of creating the illusion of a continuous tone (“CT,” “contone”) image using an output device—e.g., a printing device or other color reproduction device—capable only of binary output, for example with ink deposited or not deposited at any location on a substrate. For color printing, several images—called separations—are produced in the primary colorants, e.g., inks used to print in color, and printed on top of each other on a printing press. For typical four color printing, four images are produced in cyan (“C”), magenta (“M”), yellow (“Y”) and black (“K”), and each of these images are halftoned. Usually, digital halftoning is used and the image is recorded using an imagesetter, laser printer, ink jet printer, digital film recorder, or other output device, i.e., a color reproduction device.
The color gamut of a four-color printing process using CMYK is not very large and therefore some colors cannot be reproduced using only CMYK. Thus, processes using more than four inks have been developed in order to increase the color gamut. An example of printing with at least six inks is PANTONE's Hexachrome® system from PANTONE, Inc., Carlstadt, N.J. consisting of CMYK inks complemented with an Orange and a Green ink (CMYKOG). Another example is known as Küppers' ink set that uses CMYK, a Red, a Green, and a Blue ink (CMYKOGB). See H. Küppers: “Die Farbenlehre der Fernseh-, Foto-und Drucktechnik”, Du Mont Verlag., Köln, 1985.
Color may be specified with only three color coordinates, e.g., CIE XYZ or CIE Lab, so using six or seven or more colors for printing makes the calculation of the required ink values, e.g., the separations for any particular color rather complex. There are more values to determine —6 or more—than the three coordinates defining the color, so the problem is “underdetermined,” i.e., there are more variables to be determined than inputs. The more inks, the more difficult the determining of the separations. Thus there is a need for a method for determining good color separations when there are a large number of inks used.
Known methods for determining good color separations are often based on first measuring the colorimetric properties of patches of different amounts of a set of inks printed in sequence on top of each other. Modern color management techniques for obtaining color separations, such as COLORSYNC™ (Apple Computer, Inc., Cupertino, Calif.) and the methods promoted by the International Color Consortium (ICC, see http://www.color.org) are based on this. While these techniques can produce accurate results, and also work for halftone images, they are essentially limited to four ink systems. This is because of the large number of measurements of patches of combinations of more than 4 inks that would be needed. For example, the IT8.7.3 chart (American National Standards Institute [ANSI] Committee IT8 for Digital Data Exchange Standards) contains nearly a thousand patches for a four color output. Because of the large number of measurements, it is very difficult to characterize sets of more than four colorants, for example printing with six or seven colors, and as a consequence it is also difficult to make color separations for these printing systems based on their measured colorimetric properties. There also are applications where inks other than cyan, magenta, yellow and black need to be used, and it is not clear that prior art color management techniques that depend on measuring colorimetric properties of ink combinations work well for this. Thus there has been some effort to find better techniques applicable to more than four colorants or to different colorants.
U.S. Pat. No. 5,734,800 issued Mar. 31, 1988 to inventor Herbert, et al., titled SIX-COLOR PROCESS SYSTEM describes a system of five basic inks and a black ink. A “print grid” is printed and measured with a spectrophotometer, and a lookup table is created from this data and used to find the color separations for a specific color.
European Patent Publication EP 0586 139 A2 to inventor Litvak titled, PRINTING APPARATUS AND METHOD FOR MORE THAN FIVE COLORS describes a method for generating a print using CMYK and additional inks, such as RGB. The method is based on linear transformations of the input space, followed by non-linear corrections of the obtained CMYKRGB set and some user controlled color corrections. The Litvak method has a large number of parameters that need to be determined. Parameter values are provided in the patent for a CMYK RGB case. The parameter values depend on the particular CMYK inks being used, and, for example, different parameters would be needed for a CMYK OG case. The Litvak method appears to be an empirical method. It is not clear how to apply this method in general for generating color separations to any set of arbitrary selected inks.
U.S. Pat. No. 5,870,530 to Balasubramanian titled SYSTEM FOR PRINTING COLOR IMAGES WITH EXTRA COLORANTS IN ADDITION TO PRIMARY COLORANTS, issued Feb. 9, 1999, describes determining separations using what can be called a substitution method. A CMY to CMYKOGB example is described. The M and Y values are decreased gradually in a non-linear way and the appropriate O amount is added for colors that have an orange hue, again in a non-linear way. This apparently guarantees optimal smoothness of the separations. By using the non-linear substitution method, the possible gamut of obtainable colors is increased as compared to a simple linear substitution. While the Balasubramanian method improves on linear substitution, the gamut the method achieves is nevertheless smaller than the gamut of obtainable colors using the same ink set. For example, some saturated colors may never be obtained.
Harold Boll in “A Color to Colorant Transformation for a Seven Ink Process,” Proceeding, IS&T's Third Technical Symposium on Proofing and Printing, pp. 31–35, 1993, has described a method applicable for CMYKRGB ink sets, such as the Küppers ink set. In the Boll method, the CMYKRGB ink set is sub-divided in six groups, each group containing black and three other inks. In CMY, each ink is complementary to the combination of the other two. In the Boll method groups, none of the three non-black inks is complementary to the other two. Combinations of the different sets of inks are printed and measured using a spectrophotometer. Using the Boll method leads to an ambiguity because colors can be made using more than one of the six-ink sub sets. The way this is solved is by using the ink set where the dominant primary is maximized.
U.S. Pat. No. 5,650,942 (issued Jul. 22, 1997) and U.S. Pat. No. 5,991,511 (issued Nov. 23, 1999) to inventor Granger titled APPEARANCE-BASED TECHNIQUE FOR RENDERING COLORS ON AN OUTPUT DEVICE describe a technique that uses an appearance color space, related to the CIE XYZ color space. The color space is called VTD herein, where T and D are chromaticity coordinates and V represents some monotonic function of luminance, say lightness. An “Umbrella Surface” is defined to be the gamut boundary surface that maximizes the V-value. One might also call the umbrella surface the “Lightness Gamut Boundary” (LGB) surface. Granger uses measurements to determine the VTD values of the LGB for some particular cases such as CMYK and CMYKOG. The Granger method next constructs the 3-D gamut using the LGB and a perfect black ink, which produces a perfect neutral gray along the V-axes. The Granger method avoids the need for constructing a 3-D table that describes the color gamut of the printer in 3-D; the method uses a 2-D table to determine the values of two of the primaries from the TD-coordinates (or actually from the T/V and D/V values), and uses the V values to determine the value for black. Thus, three-dimensional interpolation is avoided.
Apparently, the method uses the maximum real surface colors gamut, also called “Pointer's Gamut” (see M. R. Pointer, “The Gamut of Real Surface Colors,” Color Research and Applications, vol. 5, no. 3, pp. 145–155, 1980) and maps this whole gamut onto the 3-D gamut constructed as described in the Granger patents.
The Granger method does not describe how to determine the umbrella surface for an arbitrary set of inks. Rather, it describes using measurements to determine the LGB for some particular cases. To be able to handle any arbitrary ink set, the number of measurements that need to be taken may be too large.
The Granger method further is restricted to the LGB surface. Sometimes another criterion is desirable. For example, if the set of inks includes two cyans, two magentas, a yellow and a black, with the chroma of the two cyans and the two magentas being more important than their lightness, then a non LGB criterion may be desirable. Furthermore, for some ink sets, the LGB may sometimes not be very smooth or continuous in terms of the ink percentages that are used to construct the surface. Thus, for these and other reasons, the Granger method may not be suitable for an arbitrary ink set.
The Granger method uses the lightest color, and does not use what can be called the “most saturated” colors of the umbrella in which one of the inks is set to 100% and two other inks are varied. This part of the umbrella may be particularly difficult to construct, so there is a need for a method that can construct it. It may be that the surfaces fold. See M. Mahy and P. Delabastita, “Inversion of the Neugebauer Equations,” Color Research And Applications, vol. 21, no. 6, pp. 365–374, 1996, and M. Mahy, “Calculation of Color Gamuts Based on the Neugebauer Model,” Color Research and Applications, vol. 22, no. 6, pp. 404–411, 1997. If there is folding then only some of the most saturated colors surface belongs to the umbrella surface, since part of the curved surface curves underneath itself, i.e., folds.
The inventor believes that the Granger method maps the entire Pointer's gamut onto the device gamut in the VTD space. The Granger method furthermore needs a neutral black production, either through a real neutral black ink or by adding one or more complementary primaries that produce a neutral black. If the darkening ink does not produce straight lines in the VTD space, the color of the separated image may become significantly different from the color in the original image.
Thus, methods known in the prior art still have one or more shortcomings and there still is a need for a separation method and apparatus for an arbitrary number of arbitrary inks that overcome one or more of the shortcomings in the prior art, and in particular, the shortcomings of the Granger Method.